In the art of electrostatic photocopying or photoprinting, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a charge corona. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light corresponding, e.g., to an optical image of an original to be copied, thereby forming an electrostatic charge pattern on the photoconductive imaging surface, i.e. a latent electrostatic image. Depending on the nature of the photoconductive surface, the latent image may have either a positive charge (e.g. on a selenium photoconductor) or a negative charge (e.g. on a cadmium sulfide photoconductor). The latent electrostatic image can then be developed by applying to it oppositely charged pigmented toner particles, which adhere to the undischarged "print" portions of the phtoconductive surface to form a toner image which is subsequently transferred by various techniques to a copy sheet (e.g. paper).
In liquid-developed electrostatic imaging, the toner particles are generally dispersed in an insulating non-polar liquid carrier, generally an aliphatic hydrocarbon fraction, which generally has a high-volume resistivity above 10.sup.9 ohm-cm, a dielectic constant below 3.0 and a low vapor pressure (less then 10 torr. at 25.degree. C.). The liquid developer system further comprises so-called charge directors, i.e. compounds capable of imparting to the toner particles an electrical charge of the desired polarity and uniform magnitude so that the particles may be electrophoretically deposited on the photoconductive surface to form a toner image. These charge director compounds are generally ionic or zwitterionic compounds which are soluble in the non polar carrier liquid. This desired charging is achieved by providing a constant optimum concentration of charge director compound in the carrier liquid, which concentration is usually determined so as to achieve the highest copy quality for the particular application.
Stable electrical characteristics of the liquid developer, in particular its bulk conductivity, are crucial to achieve high quality imaging, particularly when a large number of impressions are to be produced without changing the liquid developer system. A major factor determining the electrical characteristics of the liquid developer and affecting the electrophoretic developing process of the toner particles, is the concentration of the charge director in the carrier liquid. Thus, one of the major problems arising in liquid-developed electrostatic imaging is the variation in the charge director concentration and it is believed that many low quality copies are a result of charge director imbalance in the liquid developer system.
The application of liquid developer to the photoconductive surface clearly depletes the overall amount of liquid developer in the reservoir of an electrocopying or electroprinting machine of this type. In practice, the liquid reservoir is continuously replenished, as necessary, by addition of two liquids from two separate sources, the one providing carrier liquid and the other-a concentrated dispersion of toner particles in the carrier liquid. This is necessary in order to maintain in the carrier liquid in the reservoir a relatively constant concentration of toner particles, because the total amounts of carrier liquid and toner particles utilised per electrocopy vary as a function of the proportional area of the printed portions of the latent image on the phtoconductive surface. An original having a large proportion of printed area will cause a greater depletion of toner particles in the liquid developer reservoir, as compared to an original with a small proportion of printed area. Thus, in accordance with the aforementioned practice, the rate of replenishment of carrier liquid is controlled by monitoring the overall amount or level of liquid developer in the reservoir, whereas the rate of replenishment of toner particles (in the form of a concentrated dispersion in carrier liquid) is controlled by monitoring the concentration of toner particles in the liquid developer in the reservoir. An optical float can combine both these functions, i.e. can be utilized to monitor both the overall amount of liquid developer in the reservoir and the toner particle concentration therein.
The amount of charge director in the liquid developer reservoir must also be replenished, since the charge director is also depleted together with the carrier liquid and the toner 20 particles. In existing liquid-developed electrostatic imaging machines the charge director is replenished by adding it with the carrier liquid replenishment or with the concentrated toner dispersion. As explained hereinbelow, this results in charge director imbalance in the liquid developer system with consequent impairment of the quality of the copies.
As discussed above, the amount of toner particles utilized per electrocopy varies in proportion to the relative printed area of the image. Thus, a large number of so-called "white" copies (i.e. originals with small printed areas) will result in very small depletion of toner particles whereas the amount of carrier liquid depleted will be comparatively large. This amount of carrier liquid will be replenished and, in machines designed for adding the charge director only with the replenished carrier liquid, this will result in an increase of the concentration of charge director relative to the toner concentration. It can easily be seen that an opposite result will be observed in a photocopier machine designed so that the charge director is replenished together with the concentrated toner suspension only. In such machines a large number of "white" copies will cause a decrease in the concentration of charge director in the liquid developer system.
Similarly, a large number of "black" copies (i.e. originals with large printed areas) will cause a degradation of copy quality in opposite directions to the above. In machines wherein charge director is added with the carrier liquid only, a large number of black copies will reduce the concentration of charge director in the liquid developer, resulting in degraded copies. Against this, in machines where charge director is added to the reservoir with the concentrated toner suspension only, its concentration in the liquid developer will be increased by a larger number of black copies, resulting in lighter than optimal copies.
A possible solution to the above problem of charge director imbalance in the liquid developer would be to monitor separately the concentration of the charge director and replenish it separately from a separate source. This solution, however, is uneconomic, because it would involve the cost and complexity of providing additional measurement devices and replenishment mechanism. It follows that a simpler and more feasible solution to the problem is needed.
It is an object of the present invention to provide a solution to the problem of charge director imbalance in liquid developer systems, thereby to maintain a constant high-quality of copies in electrostatic imaging processes, independent of the "print" proportions of the originals.
Other objects and advantages of the present invention will become clear from the following description of the invention.